Incandescent lamps wear out primarily because they operate at very high temperatures which cause their tungsten filaments to evaporate and weaken. The lamps burn out when their filaments break to a weakened thin spot.
In the past, devices have been developed for increasing the life of incandescent lamps by operating the lamps below their rated voltages. The prolonged life is a result of a substantial reduction in operating temperatures occasioned by the reduced operating voltages. For example, when operating voltage is reduced 30%, the life of a 120 volt lamp will be extended about 90.5 times longer than if it were operated at its rated voltage.
U.S. Pat. No. 3,823,339, which is owned by the assignee of the present invention, discloses one of these prior art devices in the form of a rectifier diode unit which includes a thin semiconductor wafer sandwiched between a pair of thin electrical contacts. When the unit is positioned between a lamp and a lamp socket, the semiconductor wafer lies directly between a contact provided on the lamp's base and a spring leaf contact positioned in the bottom of the socket. As a result of its location, the semiconductor wafer is susceptible to breakage and thus electrical failure from the forces applied to it as the lamp is screwed into the socket. This problem becomes more serious when the semiconductor wafer and its contacts are made extremely thin so as not to raise the lamp more than about 0.030 inches above its normal position in the socket with the rectifier diode unit removed. Such a height restriction is required, in addition to many other requirements, in order to obtain U.L. approval. Thus, in spite of the overall success of the rectifier diode unit disclosed in U.S. Pat. No. 3,823,339, it does have certain problems which leave room for improvement.
Other prior art devices have been developed which utilize laterally offset rectifier assemblies including encapsulated semiconductor diodes which are provided with arcuate electrical leads (see, for instance, U.S. Pat. Nos. 3,450,893 and 3,617,766). Because such semiconductor diodes are typically encapsulated within a glass or plastic casing which has poor thermal conducting properties, almost all of the heat generated by the diodes must be conducted away through the leads. However, the leads, which in addition to having a relatively small cross-sectional area, are also totally encapsulated, thereby further limiting the ability of these devices to conduct away the heat generated by the diodes. The current carrying capability of the diodes is inhibited when they become too hot. Thus, as a result of the encapsulation of the diodes and leads, the devices disclosed in U.S. Pat. Nos. 3,450,893 and 3,617,766 are effective only on lamps having a wattage rating sufficiently low to avoid overheating of the diodes, thereby limiting the devices to use in combination with relatively low wattage lamps. Also, because the leads must be attached by, for instance, soldering to a relatively complex contact assembly (see especially U.S. Pat. No. 3,450,893), these prior art devices can be fairly expensive to manufacture.